1. Field of the Invention
This invention relates to an image scanning read-out method. This invention particularly relates to an image scanning read-out method for reading out an image stored in a sheet by two-dimensionally scanning the sheet by a light beam such as a laser beam and detecting light which is obtained by the scanning and which carries the image by use of a photomultiplier.
2. Description of the Prior Art
An image scanning read-out method using a photomultiplier has heretofore been used widely. In the method, a sheet carrying an image stored therein is two-dimensionally scanned by a light beam such as a laser beam, and then light (for example, light reflected by the sheet or light transmitted therethrough) which carries the image stored therein and which is obtained by the scanning is detected by use of a photomultiplier, thereby reading out the image.
The image scanning read-out method using a photomultiplier is employed, for example, in computer input apparatuses and image read-out apparatuses for facsimile machines. In the apparatus, image read-out is specifically carried out as described below.
A sheet carrying an image constituted by a density pattern stored therein, i.e. an original, is two-dimensionally scanned by a light beam. Light reflected by the original (in the case where the original is a paper original) or transmitted therethrough (in the case where the original is a film original) is detected and converted into a serial electric image signal by a photomultiplier. In this manner, the image carried in the original is read out. The electric image signal thus obtained is subjected to various processings, for example, transfer to a remote station via a transfer system, image processing, and storage in a magnetic recording medium.
The image scanning read-out method using a photomultiplier is employed also in a radiation image recording and reproducing system using a stimulable phosphor sheet as disclosed, for example, in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395. In this case, a stimulable phosphor sheet carrying a radiation image stored therein is exposed to stimulating rays which cause it to emit light in proportion to the radiation energy stored, and the emitted light is detected photoelectrically.
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
In the radiation image recording and reproducing system using a stimulable phosphor sheet, image read-out is specifically carried out as described below.
A stimulable phosphor sheet carrying a radiation image stored therein by being exposed to a radiation such as X-rays via an object such as the human body is two-dimensionally scanned by stimulating rays such as a laser beam which cause it to emit light in proportion to the radiation energy stored. The emitted light is detected and converted into an electric image signal by a photomultiplier. In this manner, the radiation image is read out. On the basis of the electric image signal, a visible image is reproduced in a recording material such as a photographic material or on a display device such as a cathode ray tube (CRT).
However, in the image scanning read-out apparatus using a photomultiplier as a photodetector, a very serious problem arises as described below.
The photomultiplier is suitable for detecting very weak light. However, when the photomultiplier is exposed to strong light having an intensity such that the light is clearly visible to naked eyes with a voltage being applied thereto, an excessive photocurrent flows therethrough. When this condition continues, the photomultiplier breaks. Therefore, the image scanning read-out apparatus using the photomultiplier is generally constructed so that the inside of the apparatus is completely shielded from external light, thereby preventing the photomultiplier from breaking due to external light entering the apparatus.
However, in the aforesaid method of protecting the photomultiplier, it is not always possible to prevent the photomultiplier from breaking due to external light. Namely, in the case where shielding of the inside of the apparatus from external light is not complete due to faulty assembly or when a part of the apparatus opened for adjustment or maintenance is not closed after adjustment or maintenance is finished, the photomultiplier will easily break when operations of the apparatus are started and a voltage is applied to the photomultiplier. Since this problem actually arises very often and photomultipliers are expensive, the aforesaid method is very uneconomical.
The photomultiplier can be prevented from breaking also by stopping the application of a voltage thereto when the output current thereof exceeds a predetermined value. However, in this method, application of a voltage to the photomultiplier is stopped and image read-out is interrupted not only when external light enters the apparatus but also when the intensity of light which is reflected by a sheet carrying an image stored therein, or transmitted through the sheet, or emitted by the sheet upon stimulation thereof during exposure of the sheet to a light beam and which correctly represents the image becomes high. Therefore, it is not always possible to achieve image read-out continuously, and the method is not suitable for practical use.